Hide experimental types from external source generators using internal property pattern (#1301)

src/ModelContextProtocol.Core/Protocol/CallToolRequestParams.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -33,6 +34,16 @@ public sealed class CallToolRequestParams : RequestParams
     /// When present, indicates that the requestor wants this operation executed as a task.
     /// The receiver must support task augmentation for this specific request type.
     /// </remarks>
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
+    [JsonIgnore]
+    public McpTaskMetadata? Task
+    {
+        get => TaskCore;
+        set => TaskCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
     [JsonPropertyName("task")]
-    public McpTaskMetadata? Task { get; set; }
+    internal McpTaskMetadata? TaskCore { get; set; }
 }